This invention relates to a dustproof film forming material, and more particularly it relates to a room temperature curable film forming material which can form an excellent dustproof film on the surface of a silicone elastomer.
There are various known polyorganosiloxane compositions which cure at room temperature to thereby afford an elastomer. For example, compositions prepared from polydiorganosiloxane blocked with silanol groups at both ends and an organosilicon compound which has a hydrolyzable group bonded to the silicon atom, such as acetoxy, alkoxy, dialkylketoxime, dialkylamino, dialkylaminoxy, or N-methylamido group as a crosslinking agent, have been widely used in the construction, automotive and electrical fields and the like.
These compositions are generally called "silicone sealants" and are widely used to waterproof joints of concrete or aluminum wall board, to fix a glass plate to a sash, or to bond two glass plates together in the construction industry. Demand for these silicone sealants is greatly increasing for use on high rise buildings or the like because they exhibit excellent weatherability, durability, high- and low-temperature resistance, the physical properties are scarcely altered by temperature change, there is no deterioration by ozone or ultraviolet rays, and the sealants exhibit high workability. In addition, recently some attempts have been made at utilizing them as a coating material by applying such a crosslinkable polyorganosiloxane on a roof or wall to thereby form a silicone elastomer layer having an excellent waterproofness and weatherability.
Although these silicone elastomers have the advantages described above, they are dust attracting because of the tackiness remaining after curing and primarily because of the dielectric property of the polyorganosiloxane. In particular, when non-reactive polyorganosiloxane is added to afford a flexible silicone elastomer without lowering the workability, such polyorganosiloxane that does not participate in crosslinking oozes from inside to the surface and covers the attracted dust. The resulting water-repellent dust is not washed away by rain, thereby damaging the appearance considerably.
The disadvantage of these silicone elastomers in maintaining the building's appearance, which is one of the requirements of a construction material, restricts their utilization. Furthermore, the dust attraction would damage the appearance or utility of industrial materials other than construction materials in many cases. Therefore, development of a technique for preventing the adhesion of dust to the surface of silicone elastomers has been an urgent problem in various fields including the construction industry.
A process for preventing the adhesion of dust to the surface of a silicone elastomer is disclosed in Japanese Patent Laid-Open No. 1627631/1982.
According to this process, the surface of a silicone elastomer is coated with a known coating to which dust is hard to adhere. More particularly, an alkyd resin and a room temperature curable polysiloxane composition, which consists of polydiorganosiloxane end-blocked by silanol groups and having a certain degree of polymerization and a silicon compound having a hydrolyzable group, are dissolved in an organic solvent, and the resulting composition is applied on the surface of a silicone elastomer to thereby form a dustproof film. The resulting film exhibits excellent resistance to dust. However, it has a disadvantage in that so-called cissings and unevenness would occur unless the coating process is carried out in a somewhat viscous state thereby sacrificing its workability.
As a result of investigations about these disadvantages, the present applicant has found that a film forming material prepared by adding an organic solvent containing a volatile organosilicon compound to a composition consisting of an alkyd resin and hydrolyzable silane, siloxane or optionally polydiorganosiloxane end-blocked by silanol groups, exhibits excellent workability, wetting property and adhesion to silicone elastomers at a lower viscosity, and furthermore the resulting film has excellent resistance to dust and weatherability. At first, this film forming material was prepared as a two-package formulation consisting of a principal ingredient and a curing agent which are mixed immediately prior to use. Later, there was prepared a so-called one-package film forming material having a higher workability.
It was found that a composition which can be stored in one-package form for a long time and later form a film which exhibits excellent resistance to dust is obtained by the use of methyltrimethoxysilane as the hydrolyzable silane. However, methyltrimethoxysilane has a rather low boiling point, and boils at a still lower temperature with methanol, which is a hydrolyzate thereof, in the azeotropic form. Therefore, there still remained a problem that excess methyltrimethoxysilane needed to be added to form an excellent film outdoors where air flow is much more frequent because of volatilization of the methyltrimethoxysilane, although the corresponding film forming material forms an excellent film in a closed room.
It is an object of the invention to provide a one-package dustproof film forming material without such disadvantages as described above.
As the result of my research, it was found that a film forming material which provides an excellent film when cured outdoors where air flow is much more frequent than in the case of curing in a closed room, can be prepared by adding an organotin compound as a curing catalyst to a mixture of alkyd resin and methyltrimethoxysilane, adding water to the resulting mixture to thereby induce condensation, and then removing the formed methanol from the reaction product.